Given the title, one might imagine Arena Stage’s artistic director Molly Smith deliberately timed this piece to coincide with the beginning of fall. But “The Velocity of Autumn,” which Smith also directs, is not about the calendar; it’s a poignant metaphor for the human condition.

Estelle Parsons plays Alexandra, a crusty old gal living out her days in the Brooklyn brownstone where she raised her three children. She has jury-rigged her home’s interior with barricades and strung Molotov cocktails together like party lights in order to keep the police — summoned by two of her children — from carting her off to a nursing facility. In short, she’s preparing to blow herself up and take her Park Slope neighborhood with her.

Once a successful artist, Alexandra wants to spend the rest of her life among the books and records filling her home. Her children, though, have other ideas.

The two-person play — with phone updates from the panicked siblings — is rife with oft-prickly conversations between Christopher and his mother and weighted with gallows humor. “I will set myself on fire,” she threatens, toying with an old Zippo lighter while grasping a homemade explosive. “Then I’ll bring the marshmallows,” Christopher quips, modeling his mother’s dark sense of humor.

I won’t reveal the dramatic early entry of the long-absent Christopher, who’s tasked by his overly meddlesome siblings, Michael and Jennifer, to talk their mother down from her end-of-the-world scenario. But I will say it clarifies the autumnal reference.

The story is a tender exercise in patience and reconciliation as Alexandra and her estranged son create new bonds while revealing their darkest fears and reflecting on their lives. Snappy one-liners abound, keeping the dialogue from being mawkish: “Getting you out of diapers was like the Bataan Death March!”

Parsons, best known for her Academy Award-winning performance in “Bonnie and Clyde” and as mother Bev on TV sitcom “Rosanne,” has kept her theater presence active, not only by directing but also by taking roles scripted by revered American playwrights, such as Tennessee Williams, Arthur Miller, Samuel Beckett and Edward Albee. Her portrayal of Alexandra, a woman of fierce determination, complexity and above all a wry sense of humor, proves that she is one of the country’s most brilliant — and funniest — actors.

Stephen Spinella comes to the role of Christopher with a shelf’s worth of Tony and Drama Desk awards. His sensitive performance as the wayward son returning to the fold to mitigate disaster and reconnect with his mother is genuine and deeply affecting.

Playwright Eric Coble’s “The Velocity of Autumn,” part of a trilogy of Alexandra plays, gives us a memorable night of pure, unadulterated theater that will resonate mightily — not only with caregivers and the elderly, but also for all those seeking grace and meaning in a fast-moving world.